Abstract/Summary

This study uses an analytical model, based on the cooling-to-space approximation,
and a fixed dynamical heating model to investigate the structure of the stratospheric
cooling that occurs in response to a uniform increase in stratospheric water vapour
(SWV). At all latitudes, the largest cooling occurs in the lower stratosphere and
decreases in magnitude with height. The cooling is strongly enhanced in the
Extratropics compared to the Tropics. This is markedly different to the case of
an increase in CO2, which causes maximum cooling near the stratopause and a
small warming in the tropical lower stratosphere. The qualitative differences in the
structure of the cooling can be explained by the smaller opacity of water vapour
bands in the stratosphere compared to CO2. The small opacity means that the
magnitude of the initial heating rate perturbation only decreases by a factor of four
between the upper and lower stratosphere for a SWV perturbation. Therefore, to
balance the heating rate perturbation, the largest temperature change is required
in the lower stratosphere. Increasing the background concentration of SWV causes
the water vapour bands to become more opaque. For a SWV perturbation applied
to a background SWV concentration ≥30 ppmv, the heating rate perturbation and
temperature change structurally resemble those from an increase in CO2. In the
Extratropics, the lower height of the tropopause is found to cause the enhancement
in the cooling at those latitudes. By controlling the depth of atmosphere which
adjusts to the SWV perturbation, the tropopause height affects the net exchange
of radiation between the layers in the stratosphere as they cool. The latitudinal
gradient in upwelling infrared radiation at the tropopause and variations in the
background temperature are found to have only a minor effect on the structure
of the stratospheric temperature response to a change in SWV.